This invention generally relates to synchronous transmission systems wherein digital data is transmitted over a transmission medium between a transmitter and a receiver. More particularly, the invention relates to a method of synchronizing the local clock in the receiver with the received signal so that the clock can be used to define the signal sampling instants. The invention also relates to a clock control device which utilizes said method and is located upstream of the essential, conventional circuits of the receiver such as the equalizer, the demodulator and the data detection circuit.
In a transmission system, the outgoing information is represented by certain characteristics of the signal transmitted at specific instants called signaling instants which are separated by a fixed time interval or period T. To recover such information, it is necessary that, upon receipt of the signal sent over the transmission medium, the signaling instants be identified with as much precision as possible to allow sampling of the data being transmitted at the correct instant; and the method described for such identification will be referred to hereafter as a synchronization operation.
The present invention can be used in a data receiver wherein the received signal is sampled under the control of a clock signal generator in the receiver, which generator functions to cause samples of the incoming signal to be taken as close as possible to signaling instants as determined at the receiver.
In this type of transmission receiver, the usual method of performing the synchronization operation consists in selecting a clock signal rate that will be as close as possible to the signaling rate I/T utilized at the transmitting end to define the data rate, and in then precisely adjusting the phase and the frequency of the clock signals by means of control signals transmitted before and during transmission of the data proper. Actually, the adjustment involves several operations that are practically independent: i.e., prior to the first data transmission, the system is initialized and the phase of the receiver's clock signal generator is synchronized with a received control signal, then at the beginning of each transmission, a resynchronization takes place and the phase of the clock is adjusted again; and finally, during each transmission, successive small corrections of the clock are made as a function of information extracted from the received signals.
One of the many solutions that have been proposed to solve the problem of maintaining synchronization of the clock of the receiver during data transmission consists in obtaining the information required to control said clock adjustments from a signal which is superimposed on the incoming data signal, rather than the latter signal. The major disadvantage of this solution is that it introduces a source of additional noise in the transmission.
Another prior art solution consists in sending a pilot tone on each side of the frequency spectrum of the data signal and in extracting therefrom the receiver's clock control information. The disadvantage of this solution is that it reduces the portion of the passband of the transmission medium available for data information.
It has also been proposed to extract the clock control information from the equalized data signal. However, the extraction of such information requires a number of samples per period T that is much larger than the number of samples the equalizer would need to perform the equalization function alone if it is assumed that the sampling instants at the receiving end are already synchronized with the signaling instants of the received signal.